This invention relates to a process for producing rubber-reinforced polymer compositions, and in particular to producing compositions of the type referred to as "high impact polystyrene" (HIPS) and "acrylontrile-butadiene styrene" (ABS). More particularly, the invention relates to the production of rubber-reinforced polymer compositions, in which the rubber-reinforcing particles have a bimodal particle size distribution.
Rubber-reinforced polymer compositions of the HIPS and ABS types are widely used in many applications because of their ease of molding, good gloss, and generally good mechanical properties.
It has been known for some time that improvements in mechanical properties of rubber-reinforced polymers can be achieved by providing a so-called "bimodal" distribution of rubber-reinforcing particles, i.e. one in which the rubber-reinforcing particles show two distinct peaks in their size distribution. A number of proposals have been made for ways for achieving such a bimodal particle distribution For example, U.S. Pat. No. 4,153,645 discloses a method for the preparation of a HIPS-type polymer, in which two polymer compositions are prepared, having different average particle sizes, and the two polymer compositions are then mixed by a mechanical blending process.
An alternative approach to producing HIPS and ABS polymers with a bimodal rubber distribution has been to introduce a feed stream of monomer and rubber at two different points in the polymerization system. This results in a polymer product which generally has a fairly broad spread of rubber particle sizes. Examples of this are described in EP 0 015 752, U.S. Pat. No. 4,334,039 and EP 0 096 447. A disadvantage of such methods is that the mechanical properties of the resulting product can be somewhat poor and difficult to control.
Yet a further approach is disclosed in U.S. Pat. No. 4,146,589 and EP 0 048 389. In this method, two prepolymer compositions are prepared, containing rubber particles with different particle sizes. The prepolymer compositions are then mixed and further polymerized to provide a polymer having a bimodal particle size distribution.
The method of preparation of the prepolymer compositions is described in outline in U.S. Pat. No. 4,146,589, and in detail in U.S. Pat. No. 3,903,202, referred to at column 3 line 57 of U.S. Pat. No. 4,146,589. The method consists essentially of the use of a continuously stirred tank reactor ("CSTR") for both the prepolymer compositions. The monomer is introduced continuously into such a CSTR, essentially a stirred tank, and a product stream is continuously removed. The result again is rather poor control over (a) rubber particle sizes, (b) degree of rubber grafting and (c) polymer molecular weight in the resulting product.
The polymers resulting from these polymerization methods, although superior to monomodal compositions, and to some compositions produced by mechanical blending and having a bimodal distribution, leave something to be desired in terms of mechanical properties, particularly impact resistance and it is an object of the present invention to provide a process for preparation of compositions with further improved physical property combinations.